The present invention relates to a fuel assembly for a boiling water reactor comprising a plurality of first fuel rods with a first length extending over the height of the entire fuel assembly, and a plurality of second fuel rods with a second length extending only in the lower part of the fuel assembly.
A core in a boiling water nuclear reactor comprises a plurality of vertically arranged fuel assemblies. A fuel assembly comprises a plurality of vertical fuel rods arranged between a bottom tie plate and a top tie plate in a fuel rod lattice which is usually regular. The fuel rods are retained and fixed by a number of spacers arranged in spaced relationship to each other along the fuel assembly. The fuel rods contain a column of fuel pellets arranged in a cladding tube. The fuel rods are surrounded by a fuel channel which is normally designed with a square cross section.
The core is immersed into water which serves both as coolant and as neutron moderator. During operation, the water flows upwards through the fuel assembly, part of the water thus changing into steam. Hereinafter, coolant means the water and the steam which flow through the fuel assembly. Between the fuel rods, channels are formed in which coolant may flow, so-called coolant channels. In a cross section through a fuel assembly with a regular fuel rod lattice, the coolant channels substantially have an equally large region, which is an advantage since this gives a uniform distribution of coolant flow an enthalpy in the fuel assembly.
Since the coolant in a BWR boils, a ratio of water to steam is formed which varies axially in the core. At the bottom of the core the temperature of the coolant is lower than the boiling temperature and is thus in a single-phase state, that is, only water. At the top of the core, where the coolant has reached the boiling temperature, part of the water is transformed into steam, and the coolant is therefore in a two-phase state. The further up in the core, the higher the percentage of steam in relation to the percentage of water. In the uppermost part of the core, the fuel rods are only covered with a thin film of water, outside of which steam mixed with water droplets flows, a so-called annular flow.
If the heat flow from a fuel rod becomes very large in relation to the coolant flow, there may be a risk of dryout. Dryout means that the liquid film become so thin that it is not capable of holding together, but breaks up resulting in dry wall portions, which locally leads to a considerably deteriorated heat transfer between the fuel rod and the cooling water with an ensuing greatly increased wall temperature of the fuel rod. The increased wall temperature may lead to damage with serious consequences arising on the fuel rods. Thus, it is desirable that each one of the fuel rods in a fuel assembly has an even and thick water film around the whole fuel rod. If the water film at some location of the fuel rod becomes too thin, the risk of dryout increases considerably. The risk of such local dryout determines the power that can be obtained from the whole fuel assembly.
Because of the high percentage of the steam, the pressure drop is higher in the upper part of the fuel assembly than in the lower part thereof. The greater the difference in pressure drop between the upper and lower parts of the fuel assembly, the greater is the risk of the core becoming unstable. To give the fuel assembly good stability properties, a low pressure drop in the upper part of the fuel assembly is aimed at.
When the percentage of steam rises, also the neutron moderation deteriorates in addition to the cooling, since steam is inferior to water as moderator. The reactivity of the reactor depends on the ratio between fuel and moderator. To improve the reactivity, the water/steam ratio in the upper part of the fuel assembly must be increased. One known solution to this problem is to replace some of the fuel rods with part-length fuel rods. Part-length fuel rods have a shorter axial extent than traditional full-length fuel rods and are arranged in the lower part of the fuel assembly. U.S. Pat. No. 5,229,068 discloses a fuel assembly in which the majority of the fuel rods are full-length rods, that is, they extend from the bottom tie plate to the top tie plate, and a smaller number of fuel rods are part-length rods, that is, they extend from the bottom tie plate but terminate somewhat below the top tie plate.
One problem which arises when introducing part-length fuel rods is that, in the upper part of the fuel assembly above the part-length fuel rods, large open regions with a low pressure drop are formed, which means that adjoining coolant channels, where the pressure drop is higher, are emptied of coolant which instead flows into the large open region. A consequence of this is that for the fuel rods which are disposed in a lattice position adjacent to a part-length fuel rod, the water film thins out on the rear side, that is, the side facing away from the part-length fuel rod. Consequently, the risk of dry-out is greatest on the back of the upper part of those fuel rods which are nearest the part-length fuel rods.
It is known, for example from the above-mentioned US specification, to increase the pressure drop in the large open coolant channel by various devices, thus influencing the coolant flow such that the risk of dryout decreases in the surrounding parts of the fuel bundle. The improvement achieved therefore takes place at the expense of an increased pressure drop in the upper part of the fuel assembly, which in turn leads to a deterioration of the stability properties of the fuel assembly.
The invention aims to reduce the risk of dryout of a fuel assembly in a boiling water reactor comprising both full-length and part-length fuel rods and while maintaining the stability properties of the fuel assembly.
What characterizes a fuel assembly according to the invention will become clear from the appended claims.
According to the present invention, the fuel rods adjoining the large open regions formed above the part-length fuel rods, that is, those fuel rods where the risk of dryout is greatest, are arranged such that, in their upper part, they are bent in a direction towards the centre of the large open regions such that the large open regions decrease in area while at the same time the area of adjacent coolant channels increases. In this way, the transfer of coolant from adjacent coolant channels to the large open regions decreases, whereby the supply of coolant on the back of the bent fuel rods increases and the water film increases in thickness on the back of the fuel rods. The risk of dryout is thus reduced in the fuel assembly.
One advantage of the invention is that the risk of dryout is reduced without any extra means being supplied to the fuel assembly, which contribute to increase the pressure drop in the upper part of the fuel assembly. In this way, the stability properties of the fuel assembly are maintained.
In the French patent document with publication number 2 603 416, a fuel assembly is shown which is intended for a pressurized-water reactor comprising a plurality of first fuel rods with a first length extending over the height of the whole fuel assembly and a plurality of second fuel rods with a second height extending only in the lower part of the fuel assembly, the fuel rods being arranged such that a number of coolant channels for transport of the coolant are formed between the fuel rods. In this fuel assembly the fuel rods are bent such that a regular lattice is obtained both in the upper part of the fuel assembly and in the lower part thereof. The regular lattice in the upper part of the fuel assembly has fewer positions that the lattice in the lower part of the fuel assembly. To achieve this transfer to a different regular lattice, a large majority of the fuel rods have to be bent.
With a fuel assembly according to the invention, the intention is not to achieve a regular lattice in the upper part of the fuel assembly, but to reduce the risk of dryout on certain especially exposed fuel rods. To achieve this, only these especially exposed fuel rods are bent.